common approach is to create clock generator (555 or whatever), counter, set/reset circuit to control start and stop (simple latch) and gate.

note that if clock is not 50% duty cycle, you just set double frequency and run output through flipflop. this divides frequency by two (this is why we start with double - to compensate for this) but ensures that output is perfectly 50% regardless of duty cycle of input signal (output from clock oscillator).